Thanks for your detailed response. Sorry I wasn't more detailed.
I went to the menu FILE and then to Add File to Library... as you suggested. I tried clicking one specific song/file, and then adding it, clicking the Open button. Seemed to be no problem, only once I went to the list of songs in my library, I could not find that file/song in my library. I noticed that if I went under File to Add Folder to Library, the songs were not displayed in the folder.
If I then went out of itunes and opened the specific folder for the songs/files, they were there. I tried to drag them to the library list of songs in itunes, one by one, which is what I normally do. There was something that said "link" next to the pointer as I did so. However, the songs were not added to the list of all the songs in my library. I searched for them, in Songs in my library and typing the titles in, but they weren't present.
It may or may not be relevant to add that these new songs were .mp3 files, but I have plenty of other mp3s in my itunes library. Again, I'm used to just dragging songs from another file window into my itunes library. They're in my D: drive in a music folder I made, because I don't want to add anything more to my C: drive, as I don't have much space there. So all my songs are in D drive.
I had this problem of being unable to add music long ago when originally installing itunes, but I just can't for the life of me remember what it was I did to correct it. After the recent update of itunes, I'm suddenly unable to add songs to my library. Furthermore, my original songs have been rearranged by itunes and put in new folders in my D drive, which I didn't exactly ask for. New folders in my computer for my songs just makes it more confusing.
Anyway, thanks for your time. I'm not sure what to do next. If you can think of anything, I'd be grateful to know, but either way I appreciate your efforts.